1. Field of the Invention
The present invention relates generally to electrophotographic (EP) imaging devices and, more particularly, to a method for enlarging a transfer window in an EP imaging device for toner transfer and also to a transfer station employing the method in which a layer of a thin polymer coating with high dielectric breakdown strength applied to a transfer nip defining element improves transfer efficiency and print quality.
2. Description of the Related Art
An electrophotographic (EP) imaging device uses electrostatic voltage differentials to promote the transfer of toner from component to component. During the transfer process, the toner is moved from a donating medium like a photoconductor or a transfer belt to an accepting medium, for example a belt or final media such as paper. Transfer is a core process in the entire EP printing process. The process starts when a photosensitive roll, a photoconductor, is charged and then selectively discharged to create a charge image. The charge image is developed by a developer roll covered with charged toner of uniform thickness. This developed image then travels to the first transfer process or the only transfer process in the case of direct-to-paper systems.
At first transfer the toner forming the developed image enters a nip area formed by a photoconductor roll and a transfer roll. The media for the toner to be transferred to is either a transfer belt or a transport belt supporting paper which is in between these two rolls. Time, pressure and electric fields are all critical components of the quality of the transfer process. A voltage is applied to the transfer roll to pull charged toner off the photoconductor onto the desired medium. In a two transfer system the transfer belt, now carrying the charged toner travels to a second transfer nip, similar in many ways to the first transfer nip. Again the toner is brought into contact with the medium, which it must transfer to in a nip formed by several rolls. Typically a conductive back up roll and a resistive transfer roll make up the two primary sides of the nip. As with first transfer; time, pressure and applied fields are important for high efficiency transfer.
Transfer robustness is frequently measured as the amount of voltage between the lowest voltage where acceptable transfer occurs because sufficient electric field has been built to move toner, and the highest voltage at which acceptable printing still occurs before Paschen breakdown causes undesirable print artifacts. This difference, called a transfer window, varies across environments as the receiving media varies in its properties over those same environments. The larger the difference between these two voltages, the more latitude the imaging device design has for part to part variation and still yield good quality prints.
The low end of the transfer window is determined by how well the electric field (measured in Volts/meter) can be established, and how much electric field is then required to overcome the forces of adhesion between the toner and the donating media. The high end of the transfer window is the point at which the electric field built to move the toner exceeds the Paschen limit, the limit at which the dielectric properties of the materials in the transfer nip will begin to discharge and conduct significantly more current. Breakdown almost always happens in the air gaps of the imaging device nip. Electrostatic discharge after the nip is the least severe of these as the result is to add charge to toner already transferred which might make future transfer steps more difficult. Electrostatic discharge in the nip or before the nip can cause reversal of charge on toner or movement of toner which will show up as a print defect. Thus, depending on the location of the breakdown, various print defects will likely be present in the page, which would make the print unacceptable.
Many modifications have been made to transfer systems to increase the field strength during transfer to improve transfer efficiency and print quality. These modifications include larger nip widths, increased force (pressure) in the nip and pre-wrap to bring transferring members together prior to field increase. All of these improvements have made print quality significantly better in current color (multi-toner-layer) EP imaging devices however some issues remain. Imaging devices also tend to get too much non-uniform electric field in the transfer nip which causes the system to go into overtransfer pre-maturely. This means that print quality degrades significantly, and so operating windows are compressed or disappear.
Thus, there is still a need for an innovation that will address the specific problem of overtransfer in a non-uniform electric field conditions or high conductivity conditions.